The present invention relates to a flash memory device and an operating method thereof and, more particularly, to a flash memory device related to a program operation for storing data in a memory cell and an operating method thereof.
A flash memory device is a representative non-volatile memory device which can retain data when power is off. The flash memory device can be classified into a NOR type flash memory device and a NAND type flash memory device according to the structure of a memory cell array. In the NAND type flash memory device, a memory cell array is divided into a plurality of memory cell blocks, each of which includes a plurality of strings. Each string includes a drain select transistor, a plurality of memory cells and a source select transistor, which are connected in series through junction regions (source or drain). The drain of the drain select transistor is connected to a bit line and the source of the source select transistor is connected to a common source line. The gates of the drain select transistors belonging to different strings are connected to become a drain select line, the control gates of the memory cells are connected to become a word line, and the gates of the source select transistors are connected to become a source select line. That is, memory cells included in different strings share one word line, which is defined as a page unit.
In recent years, in the NAND type flash memory device, a multi-level cell (MLC) device for storing 2-bits or more of data in one memory cell has been developed. In the case where 1-bit (0 or 1) data is stored in one memory cell, when the threshold voltage is higher than 0V, 0 data is stored in the memory cell, but when the threshold voltage is lower than 0V, 1 data is stored in the memory cell. However, in the case where four data (for example, 11, 10, 00 and 01) represented by 2 bits is stored in one memory cell, three data (for example, 10, 00 and 01) are classified as threshold voltages higher than 0V. In other words, since three data has to be represented by a threshold voltage higher than 0V within a limited range, the threshold voltages for classifying the respective data must be classified clearly. For this purpose, the respective threshold voltages must be distributed in a narrow range and the distance of the threshold voltages must be wide. If the distribution of the respective threshold voltages is wide or the distance of the threshold voltages is narrow, a malfunction may occur during a read operation.
Thus, in order to store 2-bit data, the program operation must be controlled accurately by checking program characteristics such as the program speed of a memory cell.